1. Field of the Invention
The embodiments of the invention provide a metal capping process for a back end of line (BEOL) interconnect with air gaps.
2. Description of the Related Art
Metallization patterns on integrated circuits can be formed by depositing a dielectric layer, patterning the dielectric layer by photolithography and reactive ion etching (RIE) to form a groove or trench, and depositing a metal layer that fills the trench in the dielectric layer. The metal layer typically not only fills the trenches but also covers the entire semiconductor wafer. Therefore, the excess metal is removed using either chemical-mechanical planarization (CMP) or an etch back process so that only the metal in the trenches remains.
This process, called the “damascene process,” forms conductors in-laid in the dielectric layer. The term “damascene” is derived from the name of a centuries-old process used to fabricate a type of in-laid metal jewelry first seen in the city of Damascus. In the context of integrated circuits, damascene means formation of a patterned layer imbedded on and in another layer such that the top surfaces of the two layers are coplanar. Planarity is essential to the formation of fine-pitch interconnect levels because lithographic definition of fine features is achieved using high-resolution steppers having small depths of focus. The “dual damascene” process, in which conductive lines and stud via metal contacts are formed simultaneously, is described in U.S. Pat. No. 4,789,648 issued to Chow.
As wire widths in integrated circuits continue to shrink, the electrical conductivity of the wiring material becomes increasingly more important. The material of choice since the integrated circuit art began, aluminum, is becoming less attractive than other materials, such as gold, copper, and silver, which are better conductors. In addition to processing superior electrical conductivity, these materials are more resistant than aluminum to electromigration, a property that increases in importance as wire cross-sectional areas decrease and applied current densities increase. In particular, copper is seen as an attractive replacement for aluminum because copper offers low cost, ease of processing, lower susceptibility to electromigration, and lower resistivity.
Metal caps on copper lines are the only way to improve the electromigration life time significantly. However, the selective metal cap deposition is often difficult and not manufacturable. A manufacturably feasible way to deposit metal caps on copper lines is needed.